monitoring may be necessary where it would aid in detecting early migration of contaminants

into ground water.

The basic goals of the site characterization process described in the previous chapter

are the description of the hydrogeological setting and the identification of the potential

pathways for contaminant migration. This information is the foundation for the entire

ground water monitoring program and is crucial to the proper placement of monitoring wells.

Although a monitoring system should be designed based on site specific conditions, there are

a number of practices that can be applied to ensure that detection monitoring systems satisfy

RCRA regulatory requirements. These are discussed in the following sections.

5.1.2

Placement of Point of Compliance Monitoring Wells

This section separately addresses the lateral placement and the vertical sampling

intervals of point of compliance wells. However, these two aspects of well placement should

be evaluated together in the design of the detection monitoring system. Site specific

hydrogeologic data obtained during the site characterization should be used to determine the

lateral placement of detection monitoring wells, and to select the length and vertical position

of monitoring well intakes. Potential pathways for contaminant migration are three

dimensional. Consequently, the design of a detection monitoring network that intercepts these

potential pathways requires a three dimensional approach.

The criteria for evaluating the location of point of compliance wells relative to waste

management areas are described in Section 5.1.2.1. Section 5.1.2.2 contains the

hydrogeologic criteria for evaluating lateral placement of point of compliance wells. Section

5.1.2.3 details the rationale for selection of the vertical placement and sampling intervals of

detection monitoring wells. Section 5.1.2.4 discusses the need for vadose zone monitoring.

5.1.2.1

Location of Wells Relative to Waste Management Areas

RCRA regulations for permitted facilities require point of compliance wells to be

designed and installed to detect releases of hazardous waste or hazardous waste constituents

to ground water. To meet regulatory requirements ( 264.95(a) and  264.97(a)(3)), point of

compliance monitoring wells should be installed adjacent to a hazardous waste management

unit along its downgradient limit unless the Regional Administrator has specified an alternate

point of compliance pursuant to  264.95(a)(1). In a practical sense, this means that point of

compliance monitoring wells should be as close as physically possible to the edge of

hazardous waste management unit(s), as shown in Figure 9, and screened in all transmissive

zones that may act as contaminant transport pathways. The lateral placement of monitoring

wells should be based on the number and spatial distribution of potential contaminant

migration pathways and on the depths and thicknesses of stratigraphic horizons that can serve

as contaminant migration pathways.

November 1992

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